1. Field of the Invention
The present invention relates to the quantitative determination of analytical properties of chemical systems. More particularly to a method and apparatus for using broadband, preferably near-infrared light to determine both composition and particle size of colloidal mixtures. The present invention also relates to a method and apparatus for using narrow-band or laser light with autocorrelation techniques for characterizing variations in the properties of creams and lotions.
2. Prior Art
The classical method for obtaining absorbance spectra is via direct light transmission through a transparent sample. A polychromatic beam suffers a loss of intensity at particular wavelengths as it traverses a distance L, known as the path length, through a sample. Absorption of light is a consequence of interaction with the molecules of the sample, particular species favoring specific wavelengths of light. By calculating the ratio of the transmitted beam to the initial incident beam, one obtains the transmission spectrum. Taking the negative log10 of the transmission spectrum provides the more well-known absorption spectrum. In general, these systems are well described by Beer's Law, which shows that the absorbance is proportional to the path length.
Diffuse reflectance spectroscopy is the extension of this method to non-dilute, turbid, and even opaque samples. For these types of samples, light is backscattered, i.e., directed back in the same direction as it entered. Some of the light may have been absorbed as it penetrated the sample, analogous to phenomenon in transmission. In these systems, however, the path length is determined by the amount of scattering the light has undergone. The usual theory governing such systems is that of Kubelka-Munk.
Recent work applying diffuse reflectance to physiological systems has made use of modified measurement techniques while expanding the utility of these measurements. For example, the work of Schmitt and Kumar, “Spectral Distortions in Near-Infrared Spectroscopy of Turbid Materials” Applied Spectroscopy, 50, 8, pp. 1066-1073 (1996), which is hereby incorporated by reference in its entirety, was based on a variable separation distance between the optical fiber that introduced light into the system and the one that collected it. The authors show that by departing from the traditional backscattering geometry of routine diffuse reflectance, they can detect variations of the absorption spectrum that might not have been expected. For example, they show that for a fiber separation of 6 mm, they could get an absorbance equivalent to a path length of 24 mm. They attribute this discrepancy to photon diffusion and demonstrate how to calculate the amount of absorption in such mixtures.
The critical feature that Schmitt and Kumar demonstrate is the dependence of the photon diffusion, and therefore the distortion of the spectrum away from the diffuse reflectance result, on the size of the particles doing the scattering.
Dynamic light scattering for particle size determination is usually done in dilute systems. Instead of a broadband source, laser light is used. The signal is autocorrelated to provide a measure of the fluctuation of the intensity as a function of time, usually on a microsecond—second time-scale. Particles undergo Brownian motion and the resulting dynamics impacts the intensity of the Rayleigh scattered light, generally causing a decreasing autocorrelation function which frequently appear to resemble exponential decay. When this signal is analyzed, diffusion coefficients, and inferentially, particle diameters can be deduced.